1. Field of the Invention
This invention relates to electrically conductive materials useful in the formation of EMI/RFI shielding gaskets and the like.
2. Description of the Prior Art
It is known to form EMI/RFI shielding gaskets from metal alloys such as metal phosphate and beryllium copper, the latter being the most widely used. Such alloys are relatively expensive. Moreover, in a Hazard Information Bulletin published by the Occupational Safety and Health Administration on Sep. 2, 1999, exposure to beryllium has been cited as a cause of chronic beryllium disease (CBD), a disabling and often fatal lung disease. It appears likely, therefore, that the use of alloys containing beryllium may eventually be severely curtailed, if not banned.
It is also known to form EMI/RFI gaskets from composites comprising metallic foils bonded to polymeric substrates. While such composites are less expensive than the above mentioned metal alloys, experience has indicated that either during formation into gaskets, or subsequently during repeated flexure of the gaskets, the metallic foils tend to crease, crack or pull away from the polymeric substrates.
The objective of the present invention is to provide an improved electrically conductive composite material that overcomes the shortfalls of the previously developed composites, and is both relatively inexpensive as compared to metal alloys, and readily formable into gaskets and other like EMI/RFI shielding components.
In accordance with the present invention, first and second polymeric films are provided, each being flexible and having upper and lower surfaces, with the second film being thermoformable at temperatures at and above its glass transition temperature. A flexible electrically conductive layer is applied to the upper surface of the first film, and the lower surface of the first film is adhered to the upper surface of the second film by an adhesive interlayer. The adhesive interlayer has elastic properties sufficient to accommodate relative movement between the thus adhered films occasioned by flexure of the composite. The relative movement between the first and second films during flexure of the composite serves to safeguard the electrically conductive layer from creasing, cracking or pulling away form the upper surface of the first film.
In addition to being thermoformable, the second film also may be resilient. Additional films may be included in the composite, and a second flexible electrically conductive layer may be applied to the side of the composite opposite to that to which the first conductive layer is applied.